The disappearing blocks don't have their own look because I wouldn't use them much even if I was making a longer version of the game. The one I have is to try to smooth out the learning curve. These puzzles are easier with fewer blocks in them, so removing a block is always the right call. There's no real choice to make; it's pretty much just another level.
Actually, if I had more time, one of the things I would have added is an indigo block disappearing animation. Then when you start a level, an indigo block in the shape of the previous level would disappear, to really get that nested feeling going.
This ended up being a bit long, I hope it all made sense. Thanks for playing!